I might sound nitpicky but the language is important. “Might” or “may be” or “slower than usual” are vague and don’t offer the sender all that much information about when you’re really going to respond to them. Worse, they do a horrible job of protecting the time of the email receiver who, as the responder notes, is not in the office! Such a responder implies that, not only will the vacationer reply to the email, but they may not even miss a beat. They may be slow to respond, but they also might not.
I meant email. The phone calls were similar, but a whole other problem. Your overdue notice? Did you discuss it with Head of Circulation (who knew the background)? No? Let me transfer you.

I’ve started going really, really simple on OOO messages – literally just “I’ll be out of the office until ___, please contact ___ for questions about ____”
I read it as coming from a person who was overly frustrated with their regular OOO being ignored. It’s one of my pet peeves – the OOO clearly says I’m not there and to contact Bob, yet the sender continues to reply, never contacts Bob, then blows up at me when I return that their work is delayed. I would love to be able to send something like this message in the letter but it wouldn’t fly at my office.